Newbie: execute function error!

I created a sample function:

CREATE FUNCTION concat_text (text, text) RETURNS text AS '
BEGIN   RETURN $1 || $2;
END; '
LANGUAGE 'plpgsql';


The creation was fine.  But when I execute the function with : select
concat('a', 'b');

I get the error:

NOTICE:  plpgsql: ERROR during compile of concat_text near line 1
"RROR:  parse error at or near "

What did I do wrong?

Thanks!